Fig. 310.—View of Volcanic Neck piercing and overlain by the Plateau-Basalts, Stromö, entrance of Vaagöfjord, Faroe Islands.
(From a photograph by Colonel Evans.)
The basalt-plateau here presents to the western ocean a nearly vertical escarpment which must reach a height of at least 1000 feet (see [Fig. 328]), and displays a magnificent section of the bedded lavas. The lower part of this section shows chiefly the banded structure already described, the layers of different consistency being etched out by the weather in such a way as to give them the look of stratified rocks. In the upper part of the precipice columnar and jointed or prismatic sheets are more common, but the most prominent band is the great sill, to which further reference will be made in the next Chapter.
In the course of the gradual retreat of the cliff, as the waves tunnel its base, and slice after slice is detached from its vertical front, a group of at least five small vents has been uncovered lying along a nearly north and south line. Of two of these a segment remains still on the cliff-wall and passes under the basalts; the others have been dissected and half cut away from the cliff, while groups of stacks and rocky islets of agglomerate may mark the position of others almost effaced. The horizontal distance within which the vents are crowded is probably less than half a mile, but the lofty proportions of the precipice tend to lead the eye to underestimate both heights and distances.
Fig. 311.—Section of the same Neck as that shown in [Fig. 310].
The agglomerate is a thoroughly volcanic rock, consisting of large and small blocks of various basalts, among which large slags are specially conspicuous, the whole being wrapped in a granular matrix of comminuted volcanic detritus. The arrangement of this material is best seen in the fourth vent (Figs. [310] and [311]). In this characteristic volcanic neck (b in [Fig. 311]) the boundary walls, as laid bare on the face of the precipice, are vertical, and are formed of the truncated ends of the banded lavas (a a) which have been blown out at the time of the formation of the orifice. The visible diameter of the vent was roughly estimated by me to be about 100 yards. No appreciable alteration was observed in the ends of the lavas next the vent.
The agglomerate is coarsest in the centre, where huge blocks of slaggy lava lie imbedded in the amorphous mass of compacted debris. On either side of this structureless central portion the agglomerate is distinctly stratified from the walls towards the middle, at angles of 30° to 35°. Even from a distance it can be observed that the upper limit of the agglomerate is saucer-shaped, the sloping sides of the depression dipping towards the centre of the neck at about the same angle as the rudely-stratified agglomerate underneath. From the bottom of this basin to the sea-level may be a vertical distance of some 30 yards. The basin itself has been filled up by three successive flows of basalt, of which the first (c) has merely overflowed the bottom, the second (d), entering from the northern rim of the basin, extends across to the southern slope, while the third (e), also flowing from the north, has filled up the remainder of the hollow and extended completely across it. The next succeeding lava (f) stretched over the site in such a way as to bury it entirely, and to provide a level floor for the piling up of the succeeding sheets of basalt.
| Fig. 312.—Volcanic Neck close to that shown in Figs. [310] and [311]. | Fig. 313.—Section of wall of another Neck of agglomerate in the same group with those represented in Figs. [310], [311], and [312]. |
The second vent, which is represented in Fig 312, exhibits the same features, but with some additional points of interest. It measures roughly about 20 yards in diameter at the sea-level, rises through the same group of banded basalt (a a), and is filled with a similar agglomerate (b). Its more northerly wall is now coincident with a line of fault (h) which ascends the cliff, and probably marks some subsidence after the eruptions had ceased. The southern wall shows that a dyke of basalt (g) has risen between the agglomerate and the banded basalts, and that a second dyke (g′) traverses the latter at a distance of a few feet. In this instance, also, the upper surface of the agglomerate forms a cup-shaped depression which has been filled in by two successive streams of lava (c, d). Among the succeeding lavas (e) the prominent sill (f) has been intruded, to which further allusion is made on p. 323.
These necks are obviously volcanic vents belonging to the time of the basaltic eruptions. They have been drilled through the basalts of the lower part of the cliff, but have been buried under those of the central and higher parts. The arrangement of their component materials in rude beds dipping towards the middle of each vent shows that the ejected dust and stones must have fallen back into the orifice so as to be rudely stratified towards the centre of the chimney, which was finally closed by its own last discharges of coarse detritus. The saucer-shaped upper limit of the agglomerate seems to indicate, as has been suggested above in the case of the Portree volcano, that after the eruptions ceased each vent remained as a hollow or maar on the surface of the lava-fields. And the manner in which they are filled with successive sheets of basalt shows that in course of time other eruptions from neighbouring orifices gave forth streams of lava which, in flowing over the volcanic fields, eventually buried and obliterated each of the vents.